This invention relates to a fuel injection nozzle for air-compressing, self-igniting reciprocating internal combustion engines, particularly for such engines arranged for a direct injection of fuel, comprising an axially slidable and axially guided, spring-biased valve needle for closing the nozzle orifices in the nozzle tip, and a fuel supply conduit leading into an annular chamber, which surrounds the valve needle and communicates through a transverse bore and an axial bore of the valve needle with a plenum chamber, which is defined by the valve seat cooperating with the valve needle. The valve needle is surrounded with a clearance space, which is flushed with fuel and is disposed between the plenum chamber and the means for guiding the valve needle in the nozzle tip.
A known fuel injection nozzle of that kind is disclosed in Laid-open German Application No. 27 11 350 and comprises a conical valve seat and a valve needle having a conical end portion, which cooperates with the seat and has a larger included angle than the seat. As a result, the plenum chamber is sealed from the nozzle orifices by a line seal, which is formed between the conical end portion of the valve needle and the conical valve seat of the fuel injection nozzle. In such fuel injection nozzle, a reliable seal cannot be ensured for a prolonged time because wear will inevitably occur. Besides, when the valve needle is in its closed position, the clearance space provided between the plenum chamber and the means for guiding the valve needle in the nozzle tip is separated from the plenum chamber and from the bore which is formed in the valve needle and opens into that plenum chamber when the valve needle is in its open position. As the valve needle approaches the valve seat, part of the fuel contained in the clearance space must be displaced from said clearance space. Because the approach of the valve needle to the valve seat results in a progressive decrease of the cross-section which is available for the transfer of fuel from the clearance space into the axial bore leading to the annular chamber, the rate at which fuel enters through the nozzle orifices into the combustion chamber will progressively increase so that the fuel will not be completely burnt in the combustion chamber and polluting unburnt hydrocarbons will be exhausted at an increasing rate. It must be borne in mind that the nozzle orifices are not entirely sealed in any case because the plenum chamber and the nozzle orifices are separated only by the line seal between the valve needle and the valve seat in the nozzle tip. Besides, the clearance space disposed between the plenum chamber and the means for guiding the valve needle in the nozzle tip is not continuously flushed with fuel, although such flushing is desirable for a cooling of the valve needle and the associated guiding means.